With recent development of communication technology utilizing electromagnetic waves in the microwave range for use in a portable telephone and a wireless local area network (LAN), demand for downsizing the equipment is further increasing. In order to downsize such terminal equipment, downsizing high-frequency devices constituting the equipment, such as a high-frequency filter and a resonator, is necessary.
Examples of such high-frequency devices include a laminated dielectric filter. The laminated dielectric filter is made by adequately disposing an internal electrode (conductor metal) to form a capacitor or a strip line on an internal layer of a ceramic laminated body made of a dielectric ceramic. When the same resonance mode is used, the dimension of the high-frequency device is inversely proportional to the square root of the dielectric constant (∈r) of the dielectric material to be used. Thus, fabrication of a small resonance device requires a material having a high dielectric constant.
Other characteristics required of dielectric ceramic material include a low loss in the high-frequency range, i.e. a high Q value, and a small temperature coefficient of resonance frequency (TCF). The Q value is the inverse number of dissipation loss tan δ. These characteristics allow the achievement of a high-performance filter having a low insertion loss, and excellent temperature stability.
On the other hand, an attempt is made to downsize a high-frequency device by adopting a laminated structure of a conductor metal and a dielectric ceramic composition and to obtain a high-performance device. At this time, the conductor metal is required to have a high electrical conductivity particularly in applications in the microwave range. For this reason, Ag or an alloy thereof is used in most cases. Because Ag has the highest electrical conductivity among metals, Ag is extremely advantageous in high-frequency applications. However, when a laminated structure of a conductor metal and a dielectric ceramic composition is made as described above, the conductor metal (internal electrode) and dielectric ceramic need to be fired at the same time. Thus, it is required to use a dielectric ceramic material that can densely be sintered under the firing conditions where the conductor metal to form the internal electrode neither melts nor oxidizes. In other words, it is required that the dielectric ceramic material can be fired at a temperature equal to or lower than the melting point of the conductor metal to be used, e.g. the melting point of Ag (961° C.) when Ag is used. A conventionally known example of such material is a Ba—Re—Ti—O-based material (Re being a rare-earth element) that contains various kinds of additives, glasses, or the like for lowering the sintering temperature added thereto. Such a material can provide a dielectric ceramic having a high dielectric constant, a high Q value, and a small TCF, while lowering the sintering temperature thereof (for example, see Patent Documents 1, 2, and 3).
However, the conventional art has the following problem, although the material can be sintered at approximately 900° C., at which Ag can be fired at the same time. When forming a ceramic laminated device, the ceramic material reacts with Ag during firing. This reaction makes the width or thickness of the electrode smaller than the designed value and thus hinders imparting sufficient characteristics. Further, segregation of Si or Zn, i.e. a component of the glasses or additives, or a multiple oxide thereof in the proximity of the electrode as a secondary phase is likely to decrease the Q value as a device and increase a loss.
[Patent Document 1] Japanese Patent Unexamined Publication No. H08-55518
[Patent Document 2] Japanese Patent Unexamined Publication No. H11-209172
[Patent Document 3] Japanese Patent No. 2786977